In order to address vehicle fuel efficiency standards, designers look to improve efficiencies and performance of all vehicle operating systems, including brake systems. Many vehicles employ disc brakes in their brake systems. A disc brake is a wheel brake that slows or stops rotation of the wheel by the friction caused by pushing brake pads against a brake disc with a set of calipers. Friction between the brake pads and the brake disc causes the disc and attached wheel to slow or stop. After the brakes have stopped the vehicle, the calipers are released from the brake disc. However, it has been recognized that the calipers apply a very low drag force to the disc even in the released position, for example due to the small clearance between the brake pads and the disc and irregularities in the respective surfaces of these components. It has been recognized that fuel efficiencies can be improved by understanding and addressing the residual drag between the brake disc and the brake pads in non-braking conditions.
Inertia dynamometers are used to measure torques generated at the brake during braking, and the measured torques are used in general brake design as well as in improving fuel efficiencies. Typical torque loads generated on the caliper during braking are in the range of 1,200 in-lb to 18,000 in-lb, and may be more than 40,000 in-lb. The torque sensor included in the dynamometer is selected to measure loads in a range of 0 in-lb to 50,000 in-lb with an accuracy of approximately ±50 in-lb. However, the torque sensor that is designed for measuring torque loads in this relatively high-load range is not suited for measuring torque in the relatively low load-load range with the accuracy that is required for measuring residual brake pad drag on the brake disc in a released-caliper brake configuration. For example, measurements of residual drag on the brake disc may include torque loads in the range of 0 in-lb to 225 in-lb, and are typically about 1 in-lb to 6 in-lb. Moreover, the required measurement accuracy in this range is about ±0.125 in-lb. Thus, the torque sensor used to measure active braking is not sufficiently accurate to provide useful measurements of residual drag. In order to fully investigate the operation of a single brake design, a given brake system is frequently tested on different dynamometers having sensors of differing ranges so that both loading modes can be measured with sufficient accuracy. This procedure is labor intensive and prevents understanding how different braking modes affect residual drag since the braking event measurement is performed at a different time and on a different testing machine than a residual drag measurement.